The principal objective of this research is to obtain detailed and definitive data related to the hypothesis that the irreversible phase of ischemic myocardial injury is initiated by characteristic membrane alterations which are typically associated with abnormal electrolyte flux. This objective will be accomplished by: 1) detailed study of electrolyte shifts with special emphasis on altered Ca flux; 2) evaluation of biochemical and metabolic alterations which may directly correlate with maintenance of cell integrity and 3) further evaluation of mechanisms of injury through the use of various interventions capable of protecting the ischemic myocardium. Studies will be performed in the isolated perfused rabbit interventricular septal model. Septa will be evaluated under control, ischemic and reperfused conditions. Interventions will be examined for their ability to alter the course of ischemic injury and used as a means to evaluate mechanisms of injury. Ca blockers will be examined for their ability to inhibit increased Ca influx and their ATP sparing effects. Chlorpromazine will be evaluated for its ability to attenuate the degradation of membrane phospholipids and thereby prevent increased Ca flux. Superoxide dismutase, a free radical scavenging enzyme, will be studied for its ability to protect against lipid peroxidation and subsequent membrane alterations due to superoxide generation during ischemia. Interventions will be statistically compared and the relevance of the particular mechanism to injury development analyzed. Parameters evaluated to assess reversibility or irreversibility of injury will include: physiological contractile parameters; ultrastructural evaluation; electrolyte alterations measured by atomic absorption spectrophotometry which will provide total tissue electrolytes, energy dispersive X-ray microanalysis of fresh forzen cryosections which will provide electrolyte concentrations on a cellular and subcellular level and the use of La as a cytochemical membrane probe; measurement of ATP and CP levels and ATP synthesis; measurement of total phospholipids and phospholipid components; and lipid peroxidation of cardiac membranes and cellular components. This research will provide a more detailed and definitive analysis of the significance of membrane and electrolyte alterations and the mechanisms involved in the evolution of irreversible ischemic myocardial injury.